Conventional optical information processing devices carry out recording and reproduction using optical recording media such as CD-R, CD-RW, and DVD-RAM. With these recording media, recording is carried out using light of a single wavelength and therefore recording is carried out using, for example, changes in the recording medium's refractive index due to phase changes or the like. As for recording techniques, there are single-layer and double-layer recording techniques, and recording capacity is limited by the surface area of the recording medium.
In carrying out reproduction from a recording medium, a laser light is focused on the recording medium from an external portion and miniscule indentations formed on the transparent recording medium, or changes in the reflectivity of refractive index change portions, are read, and thus the recorded information is read out.
On the other hand, in recording on the recording medium, writing is carried out by focusing light to the recording medium and causing a change such as phase change, sublimation, or perforation due to the heat at the light-focused area. The above is a recording-reproduction method based on a laser light source of one photon/one wavelength.
Furthermore, the use of recording media in a volumetric direction (volumetric recording) in order to improve the capacity of recording media is being investigated. For example, proposals and experimental manufacture have been carried out such as a technique in which information is volumetrically recorded within the surface and depth (thickness) direction of a bulk-state recording medium and an optical disk of a construction having multilayer recording layers. However, when the refractive index of each layer is different in the case of recording layers having a multilayer construction, there is a tendency for multiple interference of the laser light to occur as well as a tendency for recording interference to occur between layers. Furthermore, as the number of recording layers increases, there is less light reflected from the layers distant from the light source, and therefore it becomes difficult to obtain a sufficient S/N ratio.
Further still, when carrying out recording using single photon absorption, the recording layers are made into multiple layers and in order to absorb the laser light pertaining to the wavelength range for enabling recording, it is necessary for the recording power of the light to be extremely large when recording a layer that is distant from the light source. When the power of the laser light source for recording is increased, there is a problem known as cross erasure by which information recorded on a recording layer close to the light source is inadvertently erased when recording information on a recording layer that is distant from light source.
Recording techniques based on two-photon absorption have been proposed as a way to solve these problems. A conventional recording method using two-photon absorption is disclosed in Y. Kawata, Optics Letters, Vol. 23, No. 10, pp. 756–758, 1998 for example. In this recording method, a pulse light of a 762 nm wavelength at approximately 130 fs is used as a laser light for writing and information is recorded on a recording medium made of LiNbO3 crystal. A refractive index distribution is formed in the crystal using absorption of 381 nm wavelength light with two-photon absorption of 762 nm wavelength light. LiNbO3 is transparent with respect to light of a 762 nm wavelength and is absorptive with respect to light of a 381 nm wavelength. Two-photon absorption is produced based on a nonlinear optical effect in the focused spot of the light, which is absorbed for recording. The recording medium does not absorb light until the light power in the vicinity of the focused spot in which two-photon absorption is produced reaches a high-density state, and therefore light is absorbed in recording areas only. For this reason, the problems of absorption and cross erasure, which are problems in volumetric recording, do not occur and high-density volumetric recording becomes possible.
In conventional optical recording methods using a single wavelength and single photon, the recording density has a threshold value due to such factors as the wavelength of the light source and the NA of the recording focusing optical systems such as the condensing lens, and therefore further increases in capacity are difficult. Furthermore, there are the problems of interlayer recording interference and cross erasure when recording on a multilayer recording medium and there is the problem that there are limits to making recording layer multilayered and volumetric recording.
Furthermore, in the technique of using two-photon absorption of a single wavelength, recording is carried out with a 381 nm wavelength based on two-photon absorption using a light of a long wavelength (in conventional examples, a light of a 762 nm wavelength) as the recording light. However, when reading out the recorded bits (information), it is necessary to have a light that has a wavelength other than the wavelength of the laser light for recording. This is because the recording density is reduced if the laser light for recording is used for reading since its wavelength is long.
Furthermore, there are techniques of recording using light of two different wavelengths. When recording using a light source of two different wavelengths, the light of the two wavelengths produced from different emission apertures must be focused to the same point. For this reason, there is a problem in that complicated focusing and optical systems are required to solve issues such as correction of wavelength dispersion, focal point control, and focal point control using wavelength variation of the light source. Furthermore, a high output femtosecond laser with a peak power of several 100 W is required in order to use two-photon absorption. A large-size light source is required for this, which is a problem in that application to consumer products is difficult.